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This study employs high-brightness synchrotron X-ray diffraction for precise structural analysis of high-quality single crystals of the bilayer nickelate La₃Ni₂O₇. Using a large dynamic range detector, we successfully resolved previously overlooked weak diffraction signals, whose intensities are nearly four orders of magnitude weaker than the main Bragg reflections. These observations indicate the presence of glide mirror symmetry breaking in the crystal, leading to a polar structure (space group Ima2) instead of the previously assumed centrosymmetric model (Fmmm). Further structural refinement reveals two inequivalent nickel sites with significantly different Ni–O bond lengths. Combined with bond valence sum calculations, this suggests a checkerboard-like charge ordering of nickel sites, which, together with oxygen octahedral tilting, endows the crystal with polarity. The charge-ordered phase is structurally analogous to the polar state observed in bilayer manganites. This study establishes the polar charge-ordered state of La₃Ni₂O₇ at ambient pressure, indicating its competition with pressure-induced superconductivity, and provides critical structural insights for understanding phase competition mechanisms and the origin of pressure-induced superconductivity in bilayer nickelates.

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We report the discovery of a novel form of Ruddlesden–Popper (RP) nickelate that stands as the first example of long-range, coherent polymorphism in this class of inorganic solids. Rather than the well-known, uniform stacking of perovskite blocks ubiquitously found in RP phases, this newly discovered polymorph of the bilayer RP phase La3Ni2O7 adopts a novel stacking sequence in which single-layer and trilayer blocks of NiO6 octahedra alternate in a “1313” sequence. Crystals of this new polymorph are described in space group Cmmm, although we note evidence for a competing Imam variant. Transport measurements at ambient pressure reveal metallic character with evidence of a charge density wave transition with an onset at T ≈ 134 K. The discovery of such polymorphism could reverberate to the expansive range of science and applications that rely on RP materials, particularly the recently reported signatures of superconductivity in bilayer La3Ni2O7 with Tc as high as 80 K above 14 GPa.

This study systematically analyzes a two-site, two-orbital model of the La₃Ni₂O₇ thin-film superconductor under ambient pressure within a weakly correlated system, employing the fluctuation exchange (FLEX) approximation, with a focus on the influence of hole doping on superconducting properties. Through a detailed examination of the Fermi surface topology, it is found that when the δ pocket, formed by the d_{z²} antibonding orbital, emerges near the Γ point, the nesting between the δ and γ pockets, together with the nesting between the α and β pockets, collectively enhances s±-wave pairing at the corresponding wave vectors. The study further proposes that this enhancement mechanism of spin-fluctuation-induced pairing, driven by Fermi surface nesting, may provide a feasible route to raising the superconducting transition temperature. This work offers theoretical guidance for understanding the pairing mechanism of nickel-based thin-film superconductors under ambient pressure and for exploring higher-performance superconducting materials.